Storage stability problems are well known with liquids containing enzyme(s). Especially in enzyme-containing liquid detergents a major problem, in particular if the detergent contains protease, is that of ensuring enzyme activity over time.
The prior art has dealt extensively with improving the storage stability, for example by adding a protease inhibitor.
Boric acid and boronic acids are known to reversibly inhibit proteolytic enzymes. A discussion of the inhibition of one serine protease, subtilisin, by boronic acid is provided in Molecular & Cellular Biochemistry 51, 1983, pp. 5-32.
Boronic acids have very different capacities as subtilisin inhibitors. Boronic acids containing only alkyl groups such as methyl, butyl or 2-cyclohexylethyl are poor inhibitors with methylboronic acid as the poorest inhibitor, whereas boronic acids bearing aromatic groups such as phenyl, 4-methoxyphenyl or 3,5-dichlorophenyl are good inhibitors with 3,5-dichlorophenylboronic acid as a particularly effective one (see Keller et al, Biochem. Biophys. Res. Com. 176, 1991, pp. 401-405).
It is also claimed that aryl boronic acids which have a substitution at the 3-position relative to boron are unexpectedly good reversible protease inhibitors. Especially, acetamidophenyl boronic acid is claimed to be a superior inhibitor of proteolytic enzymes (see WO 92/19707).
The inhibition constant (K.sub.i) is ordinarily used as a measure of capacity to inhibit enzyme activity, with a low K.sub.i indicating a more potent inhibitor. However, it has earlier been found that the K.sub.i values of boronic acids do not always tell how effective inhibitors are (see for instance WO 92/19707).